The functions of the myosin alkali light chains of vertebrate fast white muscle fibers are not clear at a descriptive or molecular level. The two light chains (LC-1,LC-3) differ chiefly in the additional 41 residues at the amino terminal end of LC-1 which are unusually rich in pro, ala and lys. Of the three alkali light chain isomers possible, the two homodimers are preferred. The two light chains are uniformly distributed through the levels of organization of the fast white fiber down to the myofilament level. We have recently found evidence suggestive of an unusual degree of genetic variability in LC-1. To LC-1 alleles characteristic of different chicken strains were detected by electophoresis and peptide mapping. Variability was also detected in LC-1 at an individual level. No evidence was found for variability in LC-2 or LC-3. Preliminary evidence suggests that part if not all of the variation is occuring in the unusual amino terminal peptide present in LC-1 and not LC-3. The presence of a hypervariable region in this peptide would add information from a new direction relevant to the function of LC-1. We plan 1) to locate the variable region(s) in the two alleles described above by two dimensional peptide mapping techniques, 2) to extend the study to several other species where highly inbred strains exits, 3) to screen LC-1 of slow red and cardiac myosins, which have unusual amino terminal peptides similar to that of LC-1 of fast white fibers, for such variability. 4) in 2) and 3) where differences are found, to determine their location as in 1). Screening procedures will involve electrophoresis and one dimensional peptide mapping.